It is the simplest type of heat exchanger used in industry. It is used when the heat transfer
area required is relatively small.
It consists of concentric pipes, connecting tees, return heads and return bends. The packing glands support the inner pipe within the outer pipe. A double pipe heat exchanger arranged in two legs [i.e., when two lengths of the inner pipe are connected by a return bend] as shown in Fig. is known as a single hair-pin. Tees are provided with nozzles or screwed connections for permitting the entry and exit of the annulus fluid which crosses from one leg to the other through the return head. The return bend connects two legs of the inner pipes to each other. This exchanger can be very easily assembled in any pipe-fitting shop as it consists of standard parts and it provides inexpensive heat transfer surface. In this exchanger, one of the fluids flows through the inside pipe and the other fluid flows through the annular space created between two concentric pipes either in co-current or counter-current fashion. It is usually employed for decreasing the temperature of a hot fluid with the help of a cold fluid when flow rates are low. These are commonly used in refrigeration services.
These exchangers are usually assembled in effective lengths of 3.65, 4.57, 6 m. The distance in each leg over which the heat transfer occurs is termed as the effective length.
Fig. Schematic diagram of double pipe heat exchanger (counter current)
The major disadvantages of a double pipe heat exchanger are :
1. Small heat transfer surface in a large floor space as compared to other types (e.g., shell and tube heat exchanger).
2. Dismantling requires large time and
3. Maximum leakage points.
Apart from this, double pipe heat exchanger is very attractive where the total heat transfer surface required is small, 9.29 m2 to 14 m2 or less. It is simple in construction, cheap and easy to clean.